Although spider silk has been studied for a number of years the structures of the proteins involved have yet to be definitely determined. X-ray diffraction and solid-state nuclear magnetic resonance (NMR) were used to study major ampullate (dragline) silk from Nephila clavipes. The silk was studied in its natural state, in the supercontracted state and in the restretched state following supercontraction. The natural silk structure is dominated by beta-sheets aligned parallel to the fiber axis. Supercontraction is characterized by randomizing of the orientation of the beta-sheet. When the fiber is restretched alignment is regained. However, the same reorientation was observed for wetting of minor ampullate silk which does not supercontract. Thus, the reorientation of beta-sheets alone cannot explain the supercontraction in dragline silk. Cocoon silk showed very little beta-sheet orientation in the natural state and there were no changes upon wetting. NMR and X-ray diffraction data are consistent with the beta-sheets arising from the poly-alanine sequences known to be present in the proteins of major ampullate silk as has been proposed previously.